1/2 acre unless irrigated will not keep any grass, they will eat down and tromp it. As you are feeding hay the
1/2 acre for a dry corral for 2 horses is fine. Depending on how the sheds face, you could divide down the center of one shed, and divide the land that way making larger areas, and place the feed/forage/fodder / hay in the shed , this would give them access to shade in summer and cover for when it rains. If you keep the horses on the grass while it is wet, they will make it turn into mud faster than letting them out the grass when it has dried if that is possible. Thrush in winter and rainy seasons , just keep their hooves picked out and if on a half acre dry pen, then keep the manure picked out.
The area needed for just grazing per horse , and no hay or forage or fodder, varies for your area . many horses are kept in 10 x 10 and 12 x 12 stalls, my corrals are 18' x 32' and are big when compared to some of the boarding barns.
I have 3 horses out on a 2 1./2 irrigated pasture plus they get hay, and I will put more horses out after we redo the irrigation lines this year. keeping a horse on 1/4 acre is doable.